A method for preparing a lithographic printing plate by electrophotography is known. The method comprises uniformly charging the photoconductive layer of an electrophotographic plate making material, subjecting the charged plate making material to imagewise exposure, developing the image by either a wet or dry method to provide a toner image, fixing the toner image and treating the photoconductive layer with a desensitizing solution (etching solution) to render the nonimage area (without a toner image) hydrophilic to thereby produce a lithographic printing plate.
Electrophotographic plate making materials using a paper support are known, but a lithographic printing plate made from these materials has a printing durability such that only about 3,000 sheets can be printed. The main reason for such a low degree of printing durability is due to penetration of water into the paper substrate. Penetration of the etching solution (aqueous solution) during the desensitization treatment of the nonimage area after plate making and also penetration of dampening water during printing occur. As a result, a paper substrate absorbs water and swells, and in some cases, the paper substrate peels off from the photoconductive layer.
On the other hand, regarding image quality, for example, dot reproducibility, stable reproducibility is up to about 100 lines/inch.
One reason for this is that the water content in the support varies depending upon the temperature and humidity conditions of the atmosphere during exposure, which changes the electric conductivity of and adversely affects the photographic characteristics of the printing plate.
Various methods have been proposed to overcome these problems. One method comprises forming an intermediate layer between the paper support as a substrate and the photoconductive layer. Japanese Patent Application (OPI) No. 138904/75 discloses formation of an intermediate layer of an epoxy resin; Japanese Patent Application (OPI) No. 105580/80 discloses formation of an intermediate layer of ethylene derivatives such as ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, ethylene-vinyl acetate copolymers, ethylene-vinyl acetate-vinyl chloride copolymers and ethylene ionomers; and Japanese Patent Application (OPI) No. 14804/79 discloses formation of an intermediate layer prepared by coating and drying an aqueous polyethylene emulsion containing carbon black or graphite. However, none of the electrophotographic plate making materials using those intermediate layers provide a lithographic printing plate having excellent printing durability.